Apparatus for effecting gas liquid contact

ABSTRACT

Components, usually but not exclusively gaseous components, are removed in a liquid medium from gas streams and chemically converted into an insoluble phase or physically removed. Specifically, hydrogen sulfide may be removed from gas streams by oxidation in aqueous chelated transition metal solution in a modified agitated flotation cell. A gas-liquid contact apparatus, generally a combined chemical reactor and solid product separation device, comprising such modified agitated flotation cell also is described. In order to effect efficient mass transfer and rapid reaction, gas bubbles containing hydrogen sulfide and oxygen are formed by rotating an impeller at a blade tip velocity of at least about 350 in/sec. to achieve the required shear. To assist in the reaction, a surrounding shroud has a plurality of openings, generally of aspect ratio of approximately 1, of equal diameter and arranged in uniform pattern, such as to provide a gas flow therethrough less than about 0.02 lb/min/opening in the shroud. In general, the gas velocity index is at least about 18 per second per opening, preferably at least about 24 per second per opening. Each of the openings has an area corresponding to an equivalent diameter less than about one inch.

REFERENCE TO RELATED APPLICATION

This application is a division of copending U.S. patent application Ser.No. 622,485 filed Dec. 5, 1990 (now U.S. Pat. No. 5,174,973) which is acontinuation-in-part of U.S. patent application Ser. No. 582,423 filedSep. 14, 1990 (now abandoned) which itself is a continuation-in-part ofU.S. patent application Ser. No. 446,776 filed Dec. 6, 1989 (nowabandoned).

FIELD OF INVENTION

The present invention relates to method and apparatus for effecting theremoval of components from gas streams, in particular by chemicalconversion of gaseous components to an insoluble phase while in contactwith a liquid phase or slurry.

BACKGROUND TO THE INVENTION

Many gas streams contain components which are undesirable and which needto be removed from the gas stream prior to its discharge to theatmosphere or further processing. One such component is hydrogensulfide, while another such component is sulfur dioxide.

Hydrogen sulfide occurs in varying quantities in many gas streams, forexample, in sour natural gas streams and in tail gas streams fromvarious industrial operations. Hydrogen sulfide is odiferous, highlytoxic and a catalyst poison for many reactions and hence it is desirableand often necessary to remove hydrogen sulfide from such gas streams.

There exist several commercial processes for effecting hydrogen sulfideremoval. These include processes, such as absorption in solvents, inwhich the hydrogen sulfide first is removed as such and then convertedinto elemental sulfur in a second distinct step, such as in a Clausplant. Such commercial processes also include liquid phase oxidationprocesses, such as Stretford, LO-CAT, Unisulf, Sulferox, Hiperion andothers, whereby the hydrogen sulfide removal and conversion to elementalsulfur normally are effected in reaction and regeneration steps.

In Canadian Patent No. 1,212,819 and its corresponding U.S. Pat. No.4,919,914, the disclosure of which is incorporated herein by referencethere is described a process for the removal of hydrogen sulfide fromgas streams by oxidation of the hydrogen sulfide at a submerged locationin an agitated flotation cell in intimate contact with an iron chelatesolution and flotation of sulfur particles produced in the oxidationfrom the iron chelate solution by hydrogen sulfide-depleted gas bubbles.

The combustion of sulfur-containing carbonaceous fuels, such as fueloil, fuel gas, petroleum coke and coal, as well as other processes,produces an effluent gas stream containing sulfur dioxide. The dischargeof such sulfur dioxide-containing gas streams to the atmosphere has leadto the incidence of the phenomenon of "acid rain" which is harmful to avariety of vegetation and other life forms. Various proposals have beenmade to decrease such emissions.

A search in the facilities of the United States Patent and TrademarkOffice with respect to gas-liquid contacting procedures has revealed thefollowing United States patents as the most relevant to the presentinvention:

    ______________________________________                                        U.S. Pat. No. 2,274,658                                                                          U.S. Pat. No. 2,294,827                                    U.S. Pat. No. 3,273,865                                                                          U.S. Pat. No. 4,683,062                                    U.S. Pat. No. 4,789,469                                                       ______________________________________                                    

U.S. Pat. Nos. 2,274,658 and 2,294,827 (Booth) describe the use of animpeller to draw gas into a liquid medium and to disperse the gas asbubbles in the liquid medium for the purpose of removing dissolvedgaseous materials and suspended impurities from the liquid medium,particularly a waste stream from rayon spinning, by the agitation andaeration caused by distribution of the gas bubbles by the impeller.

The suspended solids are removed from the liquid phase by frothflotation while the dissolved gases are stripped out of the liquidphase. The process described in this prior art is concerned withcontacting liquid media in a vessel for the purpose of removingcomponents from the liquid phase.

These references contain no discussion or suggestion for removal ofcomponents from gas streams by introduction to a liquid phase. Inaddition, the references do not describe any critical combination ofimpeller--shroud parameters for effecting such removal, as requiredherein.

U.S. Pat. No. 3,273,865 describes an aerator for sewage treatment. Ahigh speed impeller in the form of a stack of flat discs forms a vortexin the liquid to draw air into the aqueous phase and circulate theaqueous phase. As in the case of the two Booth references, this priorart is concerned solely with aeration of a liquid phase to treat liquidphase components. In addition, the reference does not describe orsuggest an impeller-shroud combination for effecting such removal, asrequired herein.

U.S. Pat. No. 4,683,062 describes a perforated rotatable body structurewhich enables liquid/solid contact to occur to effect biocatalyticalreactions. This reference does not describe an arrangement in whichgas-liquid contact is effected.

U.S. Pat. No. 4,789,469 describes the employment of a series of rotatingplates to introduce gases to or remove gases from liquids. There is nodescription or suggestion of an impeller-shroud combination, as requiredherein.

Many other gas-liquid contactors and flotation devices are described inthe literature, for example:

(a) "Development of Self-Inducing Dispenser for Gas/Liquid andLiquid/Liquid Systems" by Koen et al, Proceeding of the Second EuropeanConference on Mixing, 30th March-1st April 1977;

(b) Chapter entitled "Outokumpu Flotation Machines" by K. Fallenius, inChapter 29 of "Flotation" ed M. C. Fuerstenau, AIMM, PE Inc, New York1976; and

(c) Chapter entitled "Flotation Machines and Equipment" in "FlotationAgents and Processes, Chemical Technology Review #172" M. M. Ranhey,Editor, 1980.

However, none of this prior art describes the impeller-shroud structureused herein.

SUMMARY OF INVENTION

The present invention is directed, in one embodiment, towards improvingthe process of the prior Canadian Patent No. 1,212,819 by modificationto the physical structure of the agitated flotation cell and of theoperating conditions employed therein, so as to improve the overallefficiency and thereby decrease operating and capital costs, while, atthe same time, retaining a high efficiency for removal of hydrogensulfide from the gas stream.

However, the present invention is not restricted to effecting theremoval of hydrogen sulfide from gas streams by oxidation, but ratherthe present invention is generally applicable to the removal of gas,liquid and/or solid components from a gas stream by chemical reaction,and more broadly relates to the removal of components of any physicalform as well as sensible heat from a gas stream by gas-liquid contact.

In one embodiment of the present invention, an efficient contact of gasand liquid is carried out for the purpose of effecting a reaction whichremoves a component of the gas and converts that component to aninsoluble phase while in contact with the liquid phase. More broadly, agas stream is brought into contact with a liquid phase in such a mannerthat there is efficient contact of the gas stream with the liquid phasefor the purpose of removing components from the gas stream. For example,the removal of a component may be effected by a physical separationtechnique, rather than a chemical reaction. These operations contrastmarkedly with the conventional objective of the design of a flotationcell, which is to separate a slurry or suspension into a concentrate anda gangue or barren stream in minerals beneficiation. A component is notspecifically removed from a gas stream during the latter operations.

There are a variety of processes to which the principles of the presentinvention can be applied. The processes may involve reaction of agaseous component of the gas stream with another gaseous species in aliquid phase, usually an aqueous phase, often an aqueous catalystsystem.

One example of such a process is the oxidative removal of hydrogensulfide from gas streams in contact with an aqueous transition metalchelate system to form sulfur particles, as described generally in theabove-mentioned Canadian Patent No. 1,212,819.

Another example of such a process is in the oxidative removal ofmercaptans from gas streams in contact with a suitable chemical reactionsystem to form immiscible liquid disulfides.

A further example of such a process is the oxidative removal of hydrogensulfide from gas streams using chlorine in contact with an aqueoussodium hydroxide solution, to form sodium sulphate, which, after firstsaturating the solution, precipitates from the aqueous phase.

An additional example of such a process is the removal of sulfur dioxidefrom gas streams by the so-called "Wackenroder's" reaction by contactinghydrogen sulfide with an aqueous phase in which the sulfur dioxide isinitially absorbed, to form sulfur particles. This process is describedin U.S. Pat. Nos. 3,911,093 and 4,442,083. The procedure of the presentinvention also may be employed to effect the removal of sulfur dioxidefrom a gas stream into an absorbing medium in an additional gas-liquidcontact vessel.

A further example of such a process is the removal of sulfur dioxidefrom gas streams by reaction with an aqueous alkaline material.

The term "insoluble phase" as used herein, therefore, encompasses asolid insoluble phase, an immiscible liquid phase and a component whichbecomes insoluble when reaching its solubility limit in the liquidmedium after start up.

The component removed from the gas stream usually is a gaseous componentbut the present invention includes the removal of other components fromthe gas stream, such as particulate material or dispersed liquiddroplets.

For example, the present invention may be employed to remove solidparticles or liquid droplets from a gas stream, i.e. aerosol droplets,such as by scrubbing with a suitable liquid medium. Similarly, moisturemay be removed from a gas stream, such as by scrubbing with a suitablehydrophilic organic liquid, such as glycol.

A wide range of particle sizes from near molecular size through Aikinnuclei to visible may be removed from a gas stream by the wellunderstood mechanisms of diffusion, interception, impaction and capturein a foam layer.

More than one component of any type and components of two or more typesmay be removed simultaneously or sequentially from the gas stream. Inaddition, a single component may be removed in two or more sequentialoperations.

The present invention also may be employed to remove sensible heat (orthermal energy) from a gas stream by contacting the gas stream with asuitable liquid phase of lower temperature to effect heat exchange.Similarly, sensible heat may be removed by evaporation of a liquidphase.

Accordingly, in one aspect of the present invention, there is provided amethod of removing a component from a gas stream containing the same ina liquid phase, comprising a plurality of steps. A component-containinggas stream is fed to an enclosed gas-liquid contact zone in which islocated a liquid medium.

An impeller comprising a plurality of blades is rotated about agenerally vertical axis at a submerged location in the liquid medium soas to induce flow of the gas stream along a generally vertical flow pathfrom external to the gas-liquid contact zone to the submerged location.

The impeller is surrounded by a shroud through which are formed aplurality of openings, generally within a preferred range of impeller toshroud diameter ratios found in flotation cells. The impeller is rotatedat a speed corresponding to a blade tip velocity of at least about 350in/sec., preferably about 500 to about 700 in/sec., so as to generatesufficient shear forces between the impeller blades and the plurality ofopenings in the shroud to distribute the gas stream as fine gas bubblesof diameter no more than about 1/4 inch, in the liquid medium, therebyachieving intimate contact of the component and liquid medium at thesubmerged location so as to effect removal of the component from the gasstream into the liquid medium.

Materials are permitted to flow from the interior of the shroud throughthe openings therein into the body of the liquid medium external to theshroud at a gas velocity index at approximately atmospheric pressure ofat least about 18 per second per opening, preferably at least about 24per second per opening, whereby any removal of component not effected inthe interior of the shroud is completed in the region adjacent to theexterior of the shroud. The gas velocity index more preferably is atleast about 30 per second per opening, and may range to very highvalues, such as up to about 400 per second per opening, and often is inexcess of about 100 per second per opening.

The gas velocity index per opening is determined by the relationship:##EQU1## where the equivalent diameter is determined by therelationship: ##EQU2##

A component-depleted gas stream is vented from a gas atmosphere abovethe liquid level in the gas-liquid contact zone to exterior of theenclosed gas-liquid contact zone.

While the gas-liquid contact procedure is generally operated with theenclosed reaction zone operating at or near atmospheric pressure, italso is possible to carry out the method under superatmospheric andsubatmospheric conditions.

While the present invention, in its method aspect, is describedspecifically with respect to the removal of hydrogen sulfide and sulfurdioxide from gas streams containing the same by reaction to form sulfurand recovery of the so-formed sulfur by flotation, it will be apparentfrom the foregoing and subsequent discussion that both the apparatusprovided in accordance with a further aspect of the present inventionand the method aspect of the invention are useful for effecting otherprocedures where a component of a gas stream is removed in a liquidmedium.

In one preferred aspect of the invention, hydrogen sulfide is convertedto solid sulfur particles by oxygen in an aqueous transition metalchelate solution as a reaction medium. The oxygen is present in anoxygen-containing gas stream which is introduced to the same submergedlocation in the aqueous catalyst solution as the hydrogensulfide-containing gas stream, either in admixture therewith or as aseparate gas stream. The oxygen-containing gas stream similarly isdistributed as fine bubbles by the rotating impeller, which achievesintimate contact of oxygen and hydrogen sulfide to effect the oxidation.The hydrogen sulfide, therefore, is removed by chemical conversion toinsoluble sulfur particles.

The solid sulfur particles are permitted to grow or are subjected tospherical agglomeration or flocculation until they are of a size whichenables them to be floated from the body of the reaction medium byhydrogen sulfide-depleted gas bubbles.

The sulfur is of crystalline form and particles of sulfur aretransported when having a particle size of from about 10 to about 50microns in diameter from the body of reaction medium by the hydrogensulfide-depleted gas bubbles to form a sulfur froth floating on thesurface of the aqueous medium and a hydrogen sulfide-depleted gasatmosphere above the froth, from which is vented a hydrogensulfide-depleted gas stream. The sulfur-bearing froth is removed fromthe surface of the aqueous medium to exterior of the enclosed reactionzone.

In another preferred aspect of the present invention, sulfur dioxide isreacted with an alkaline medium to remove the sulfur dioxide from a gasstream bearing the same. Sulfur dioxide is absorbed from the gas streaminto the aqueous alkaline medium and reacts with active alkali thereinto form salts, with the sulfur dioxide-depleted gas stream being ventedfrom the reaction medium.

According to another aspect of the present invention, there is providedgas-liquid contact apparatus comprising an enclosed tank means. Inletgas manifold means is provided for feeding at least one gas streamthrough an inlet in an upper closure to the tank means. Standpipe meanscommunicates with the inlet and extends downwardly within the tank fromsaid upper closure.

Impeller means comprising a plurality of blades is located towards thelower end of said standpipe means and is mounted to a shaft for rotationabout a generally vertical axis. Drive means is provided for rotatingthe shaft.

Shroud means surrounds the impeller means and has a plurality ofopenings, which may be equal diameter and arranged in a uniform pattern,and extending through the wall of the shroud means. Each of the openingsthrough the shroud means has an equivalent diameter, as defined above,of generally less than about 1 inch. However, for large capacity units,the openings may have a larger equivalent diameter. In general, theopenings have an equivalent diameter related to the impeller diametersuch that the ratio of equivalent diameter of opening to impellerdiameter is less than about 0.15. By modification to the shroud in thisway, the apparatus can be operated to provide a gas velocity index of atleast 18 per second per opening. Vent means from the tank means also isprovided.

The device means for rotating the shaft generally comprises an externaldrive motor. However, the drive means may comprise an in-line impellerdriven by the pressure of the gas stream being treated.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an upright sectional view of a novel gas-liquid contactapparatus provided in accordance with one embodiment of the invention;

FIG. 2 is a detailed perspective view of the impeller and shroud of theapparatus of FIG. 1; and

FIG. 3 is a close-up perspective view of a portion of the shroud of FIG.2.

GENERAL DESCRIPTION OF INVENTION

One embodiment of the present invention is directed towards removinghydrogen sulfide from gas streams. High levels of hydrogen sulfideremoval efficiency are attained, generally in excess of 99.99%, from gasstreams containing any concentration of hydrogen sulfide. Residualconcentrations of hydrogen sulfide less than 0.1 ppm by volume can beattained.

The process of the invention is able to remove effectively hydrogensulfide from a variety of different source gas streams containing thesame, provided there is sufficient oxygen to oxidize the hydrogensulfide. The oxygen may be present in the hydrogen sulfide-containinggas stream to be treated or may be separately fed, as is desirable wherenatural gas or other combustible gas streams are treated.

Hydrogen sulfide-containing gas streams which may be processed inaccordance with the invention include fuel gas and natural gas and otherhydrogen sulfide-containing streams, such as those formed in oilprocessing, oil refineries, mineral wool plants, Kraft pulp mills, rayonmanufacturing, heavy oil and tar sands processing, coking coalprocessing, meat rendering, a foul gas stream produced in themanufacture of carborundum and gas streams formed by air strippinghydrogen sulfide from aqueous phases. The gas stream may be onecontaining solids particulates or may be one from which particulates areabsent. The ability to handle a particulate-laden gas stream in thepresent invention without plugging may be beneficial, since thenecessity for upstream cleaning of the gas is obviated.

The process of the present invention for effecting removal of hydrogensulfide from a gas stream containing the same employs a transition metalchelate in aqueous medium as the catalyst for the oxidation of hydrogensulfide to sulfur. The transition metal usually is iron, although othertransition metals, such as vanadium, chromium, manganese, nickel andcobalt may be employed. Any desired chelating agent may be used butgenerally, the chelating agent is ethylenediaminetetraacetic acid(EDTA). An alternative chelating agent is HEDTA. The transition metalchelate catalyst may be employed in hydrogen or salt form. The operativerange of pH for the process generally is about 7 to about 11.

The hydrogen sulfide removal process of the invention is convenientlycarried out at ambient temperatures of about 20° to 25° C., althoughhigher and lower temperatures may be adopted and still achieve efficientoperation. The temperature generally ranges from about 5° to about 80°C.

The minimum catalyst concentration to hydrogen sulfide concentrationratio for a given gas throughput may be determined from the rates of thevarious reactions occurring in the process and is influenced by thetemperature and the degree of agitation or turbulence in the reactionvessel. This minimum value may be determined for a given set ofoperating conditions by decreasing the catalyst concentration until theremoval efficiency with respect to hydrogen sulfide begins to dropsharply. Any concentration of catalyst above this minimum may be used,up to the catalyst loading limit of the system.

The removal of hydrogen sulfide by the process of the present inventionis carried out in an enclosed gas-liquid contact zone in which islocated an aqueous medium containing transition metal chelate catalyst.A hydrogen sulfide-containing gas stream and an oxygen-containing gasstream, which usually is air but may be pure oxygen or oxygen-enrichedair, are caused to flow, either separately or as a mixture, along avertical flow path from outside the gas-liquid contact zone to asubmerged location in the aqueous catalyst medium, from which themixture is forced by a rotating impeller to flow through the shroudopenings into the body of the aqueous medium. The impeller comprises aplurality of outwardly-extending blades and is rotated about a generallyvertical axis. The rotating impeller also draws the liquid phase to thelocation of introduction of the gas streams from the body of aqueousmedium in the enclosed zone.

The gas streams are distributed as fine bubbles by the combined actionof the rotating impeller and a surrounding shroud which has a pluralityof openings therethrough. To achieve good gas-liquid contact and henceefficient oxidation of hydrogen sulfide to sulfur, the impeller isrotated rapidly so as to achieve a blade tip velocity of at least about350 in/sec, preferably about 500 to about 700 in/sec. In addition, shearforces between the impeller and the stationary shroud assist inachieving the good gas-liquid contact by providing a gas velocity indexwhich is at least about 18 per second per opening, preferably at leastabout 24 per second per opening. Other than at or near the upper limitof capacity of a unit, the gas flow rate through the openings is lessthan about 0.02 lb/min/opening in the shroud, generally down to about0.004, and preferably in the range of about 0.005 to about 0.007lb/min/opening in the shroud.

The distribution of the gases as fine bubbles in the reaction medium inthe region of the impeller enables a high rate of mass transfer to occurin the catalyst solution, a complicated series of chemical reactionsoccurs resulting in an overall reaction which is represented by theequation:

    H.sub.2 S+1/2O.sub.2 →S+H.sub.2 O

The overall reaction thus is oxidation of hydrogen sulfide to sulfur.

The solid sulfur particles grow in size until of a size which can befloated. Alternative procedures of increasing the particle size may beemployed, including spherical agglomeration or flocculation. Theflotable sulfur particles are floated by the hydrogen sulfide-depletedgas bubbles rising through the body of catalyst solution and collectedas a froth on the surface of the aqueous medium. The sulfur particlesrange in size from about 10 to about 50 microns in diameter and are incrystalline form.

The series of reactions which is considered to occur in the metalchelate solution to achieve the overall reaction noted above is asfollows: ##EQU3##

Alternatively, the oxygen-containing gas stream may be introduced to themetal chelate solution at a different submerged location from thehydrogen sulfide-containing air stream using a second impeller/shroudcombination.

Another embodiment of the invention is directed towards removing sulfurdioxide from gas streams. The procedure shows many similarities with thehydrogen sulfide-removal procedure just described, except that theaqueous medium contains an alkaline material.

The aqueous alkaline medium into which the sulfur dioxide-containing gasstream is introduced may be provided by any convenient alkaline materialin aqueous dissolution or suspension. One convenient alkaline materialwhich can be used is an alkali metal hydroxide, usually sodiumhydroxide. Another convenient material is an alkaline earth metalhydroxide, usually a lime slurry or a limestone slurry.

Absorption of sulfur dioxide in an aqueous alkaline medium tends toproduce the corresponding sulfite. It is preferred, however, that thereaction product be the corresponding sulfate, in view of the greatereconomic attraction of the sulfate salts. For example, where lime orlimestone slurry is used, the by-product is calcium sulfate (gypsum), amulti-use chemical.

Accordingly, in a preferred aspect of the invention, anoxygen-containing gas stream, which usually is air but which may be pureoxygen or oxygen-enriched air, analogously to the case of hydrogensulfide, also is introduced to the aqueous alkaline reaction medium, soas to cause the sulfate salt to be formed. When such oxidation reactionis effected in the presence of a lime or limestone slurry, it isgenerally preferred to add a small amount of an anti-caking agent, toprevent caking of the by-product calcium sulfate on the lime orlimestone particles, decreasing their effectiveness. One suitableanti-caking agent is magnesium sulfate.

The concentration of sulfate salt builds up in the aqueous solutionafter initial start up until it saturates the solution, whereupon thesulfate commences to precipitate from the solution. The crystallinesulfate, usually sodium sulfate or calcium sulfate crystals, may befloated from the solution by the sulfur dioxide depleted gas bubbles, ifdesired, with the aid of flotation-enhancing chemicals, if required.

The oxygen-containing gas stream, when used, may be introduced to theaqueous medium at the same submerged location as the sulfurdioxide-containing gas stream, either in admixture with the sulfurdioxide-containing gas stream or as a separate gas stream.

Alternatively, the oxygen-containing gas stream may be introduced to theaqueous alkaline medium at a different submerged location from thesulfur dioxide-containing gas stream using a second impeller/shroudcombination.

The process of the invention is capable of rapidly and efficientlyremoving sulfur dioxide from gas streams containing the same. Such gasstreams may contain any concentration of sulfur dioxide and the processis capable of removing such sulfur dioxide in efficiencies exceeding99.99%. Residual sulfur dioxide concentrations below 0.1 ppm by volumecan be achieved.

This sulfur dioxide removal embodiment of the invention can be carriedout under a variety of process conditions, the choice of conditionsdepending, to some extent, on the chemical imparting alkalinity to thereaction medium. For an alkali metal hydroxide, the aqueous alkalinesolution generally has a concentration of from about 50 to about 500g/L. For an alkaline earth metal hydroxide, the aqueous alkalinesolution generally has a concentration of from about 1 to about 20 wt %.The active alkalinating agent may be continuously and intermittentlyreplenished to make up for the conversion to the corresponding sulfiteor sulfate. The reaction temperature may vary widely from about 5° toabout 80° C.

DESCRIPTION OF PREFERRED EMBODIMENT

Referring to the drawings, a novel gas-liquid contact apparatus 10,provided in accordance with one embodiment of the invention, is amodified form of an agitated flotation cell. The design of thegas-liquid contactor 10 is intended to serve the purpose of efficientlycontacting gases to effect removal of a component of the gas, such as byreaction to produce a flotable insoluble phase. This design differs fromthat of an agitated flotation cell whose objective is to separate aslurry or suspension into a concentrate and a gangue or barren stream.

There are significant differences between a conventional agitatedflotation cell and the modified flotation cell 10 of the presentinvention which arise from the differences in requirements of the twodesigns. In the present invention, the substances which are treated arecontained in the gas stream whereas, in an agitated flotation cell, thesubstances which are treated are contained within the slurry and the gasis employed to float the particles out of the slurry.

An agitated flotation cell is designed to process a slurry orsuspension. The capacity of the cell is measured as the volume oftreated slurry in a given time and the efficiency is measured as themass fraction of desired mineral separated relative to that in theentering slurry or suspension. Normally, a number of stages is required,including a roughing stage to effect the non-reactive separation. Incontrast, an apparatus which removes a component from a gas stream bychemical reaction or physical separation, as in the case of device 10,is engineered to process and treat a flow of gas. Capacity is measuredin volume of gas throughput and efficiency is measured in terms of therelative removal as compared to the desired removal. Normally, only oneseparation step is required.

In addition, an agitated flotation cell is designed to generate amultiplicity of small air bubbles which are distributed uniformly bymeans of a shroud to ensure good contacting between gas bubbles and thedesired mineral particles. Normally, no chemical reaction takes place inthe cell but surface-active agents may be added to change theflotability of the concentrate. In contrast, in a chemical reactor, suchas device 10, the contacting and reaction chemistry are of paramountimportance and directly affect the efficiency of the unit. Effectivecontacting between gas phase and liquid phase is achieved in the presentinvention to effect chemical and physical separation operation byrotation of the impeller at rates well in excess of those used in anagitated flotation cell. The reactor 10 as an H₂ S reactor utilizes achemical reaction in which hydrogen sulfide is oxidized through themedium of a catalyst by oxygen. The flotation of sulfur is a verysignificant additional benefit in the operation of the reactor but isnot a primary design criterion.

In a conventional agitated flotation cell, the impeller is smallrelative to the size of the flotation cell, since its purpose is toproduce a myriad of small bubbles and not to promote efficientgas-liquid contacting. The shroud is designed with relatively few largeopenings to distribute the small bubbles uniformly in the cell, ensuringgood contacting between the bubbles and the desired contacting phase.The bubbles are maintained within a relatively narrow size range toensure a large surface area for gas-solid contacting, not gas-liquidcontacting, and the bubbles are active throughout the entire volume ofthe cell. As cells increase in size, the proportion of liquid pumpedthrough the shroud increases and the momentum of the liquid carries thebubbles required for flotation to the outer reaches of the cell.

In contrast, in the gas-liquid contactor herein, the impeller may belarger relative to the size of the reactor and its design may be alteredto increase the efficiency of gas-liquid contacting. Most of thechemical or physical process occurs very close to the impeller, so thatthe effective zone is a much smaller fraction of cell volume than in thecase of flotation where separation in the bulk is required. The shroudis designed with a large number of smaller openings, which usually havesharp edges (i.e. the surfaces intersect at an acute angle) to promotesecondary contacting by which gas shearing further improves theefficiency of the reaction.

In the apparatus 10 of the invention, the gas inlets and outlets aremuch larger than in a conventional flotation cell to accommodate anincreased flow of gas. Similarly, liquid inlets and outlets aresufficient for the purposes of filling and draining the vessel, but notfor the continuous flow of slurry as in the case of the agitatedflotation cell.

The reactor 10, constructed in accordance with one embodiment of theinvention and useful in chemical and physical processes for removing acomponent from a gas stream, such as oxidative removal of hydrogensulfide, comprises an enclosed housing 12 having a standpipe 14extending from exterior to the upper wall 16 of the housing 12downwardly into the housing 12. Inlet pipes 18,20 communicate with thestandpipe 14 through an inlet manifold at its upper end for feeding ahydrogen sulfide-containing gas stream and air to reactor 10.

The inlet pipes 18,20 have inlet openings 22,24 through which the gasflows. The openings are designed to provide a low pressure drop.

Generally, the flow rate of gas streams may range upwardly from aminimum of about 50 cu.ft/min., for example, in excess of about 500cu.ft/min., although much higher or lower flow rates may be employed,depending on the intended application of the process. The pressure dropacross the unit may be quite low and may vary from about -5 to about +10in. H₂ O, preferably from about 0 to less than about 5 in. H₂ O. Forlarger units employing a fan or a blower to assist the gas flow rate tothe impeller, the pressure drop may be greater.

A shaft 26 extends through the standpipe 14 and has an impeller 28mounted at its lower end just below the lower extremity of the standpipe14. A drive motor 30 is mounted to drive the shaft 26. Although there isillustrated in the drawings an apparatus 10 with a single impeller 28,it is possible to provide more than one impeller and hence more than oneoxidative reaction (or other chemical or physical process) location inthe same enclosed tank. The gas flow rate to the reactor referred toabove represents the flow rate per impeller.

The impeller 28 comprises a plurality of radially-extending blades 32.The number of such blades may vary and generally at least four bladesare employed, with the individual blades being equi-angularly spacedapart. The impeller is illustrated with the blades 32 extendingvertically. However, other orientations of the blades 32 are possible.

Generally, the standpipe 14 has a diameter dimension related to that ofthe impeller 28 and the ratio of the diameter of the standpipe 14 tothat of the impeller 28 generally may vary from about 1:1 to about 2:1.However, the ratio may be lower, if the impeller is mounted below thestandpipe. The impeller 28 generally has a height which corresponds toan approximately 1:1 ratio with its diameter, but the ratio generallymay vary from about 0.3:1 to about 3:1. As the gas is drawn down throughthe standpipe 14 by the action of the rotary impeller 28 and the liquidphase is drawn into the impeller, the action of gas and liquid flows androtary motion produce a vortex of liquid phase in the upper region ofthe impeller 28.

The ratio of the projected cross-sectional area of the shrouded impeller28 to the cross-sectional area of the cell 10 may vary widely, and oftenis less but may be more than in a conventional agitated flotation cell,since the reaction is confined to a small volume of the reaction mediumand will be determined by the ultimate use to which the apparatus 10 isput. The ratio may be as little as about 1:2. However, where additionalprocessing of product is required to be effected efficiently, such asflotation of sulfur, the ratio generally will be higher.

Another function of the impeller 28 is to distribute the induced gasesas small bubbles. This result is achieved by rotation of the impeller28, resulting in shear of liquid and gases to form fine bubblesdimensioned no more than about 1/4 inch. A critical parameter indetermining an adequate shearing is the velocity of the outer tip of theblades 32. A blade tip velocity of at least about 350 in/sec is requiredto achieve efficient (i.e., 99.99%+) removal of hydrogen sulfide,preferably about 500 to about 700 in/sec. This blade tip velocity ismuch higher than typically used in a conventional agitated flotationcell, wherein the velocity is about 275 in/sec.

The impeller 28 is surrounded by a cylindrical stationary shroud 34having a uniform array of circular openings 36 through the wall thereof.The shroud 34 generally has a diameter slightly greater than thestandpipe 14. Although, in the illustrated embodiment, the shroud 34 isright cylindrical and stationary, it is possible for the shroud 34 topossess other shapes. For example, the shroud 34 may be tapered, withthe impeller 28 optionally also being tapered. In addition, the shroud34 may be rotated, if desired, usually in the opposite direction to theimpeller 28.

Further, the openings 36 in the shroud are illustrated as beingcircular, since this structure is convenient. However, it is possiblefor the openings to have different geometrical shapes, such as square,rectangular or hexagonal. Further, all the openings 36 need not be ofthe same shape or size.

The shroud 34 serves a multiple function in the device. Thus, the shroud34 prevents gases from by-passing the impeller 28, assists in theformation of the vortex in the liquid necessary for gas induction,assists in achieving shearing and maintains the turbulence produced bythe impeller 28. The effect of the impeller-shroud combination may beenhanced by the employment of a series of elongate baffles, provided onthe internal wall of the shroud 34, preferably vertically extending fromthe lower end to the upper end of the openings in the shroud.

The shroud 34 is spaced only a short distance from the extremity of theimpeller blades 32, in order to provide the above-noted functions.Generally, the ratio of the diameter of the shroud 34 to that of theimpeller 28 generally is about 2:1 to about 1.2:1, preferablyapproximately 1.5:1.

In contrast to the shroud in a conventional agitated flotation cell, theopenings 36 generally are larger in number and smaller in diameter, inorder to provide an increased area for shearing, although an equivalenteffect can be achieved using openings of large aspect ratio, such asslits. When such circular openings are employed, the openings 36generally are uniformly distributed over the wall of the shroud 34 andusually are of equal size. The equivalent diameter of the openings 36often is less than about one inch and generally should be as small aspossible without plugging, preferably about 3/8 to about 5/8 inch indiameter, in order to provide for the required gas flow therethrough.When the openings 36 are of non-circular geometrical shape and of aspectratio which is approximately unity, then the area of each such opening36 generally is, less than the area of a circular opening having anequivalent diameter of about one inch, preferably about 3/8 to about 5/8inch. The openings have sharp corners to promote shearing.

The openings 36 are dimensioned to permit a gas flow rate therethroughcorresponding to less than about 0.02 lb/min/shroud opening, generallydown to about 0.004 lb/min/shroud opening. As noted earlier, the gasflow rate may be higher at or near the upper limit of capacity of theunit. Preferably, the gas flow rate through the shroud openings is about0.005 to about 0.007 lb/min/opening in the shroud. As noted above, ingeneral, the gas velocity index is at least about 18 per second peropening in the shroud, preferably at least about 24 per second peropening, and more preferably at least about 30 per second per opening.

As a typical example, in a conventional agitated flotation cell,forty-eight circular openings 1.25 inches in diameter for acircumferential length of 188 inches may be employed while, in the samesize unit constructed as a reactor in accordance with the presentinvention, 670 circular openings each 5/8 -inch in diameter are used fora total circumferential length of 789 inches. In addition, in thepresent invention the gas flow through the openings is typically 0.007lb/min/opening (a gas velocity index of 65 per second per opening) inthe shroud, while in a conventional agitated flotation cell of the sameunit size the same parameter is 0.03 lb/min/opening (a gas velocityindex less than 10 per second per opening) in the shroud. As may be seenfrom this typical comparison, the physical dimensions of the openingsand the gas flow are significantly different in the gas-liquid contactdevice of this invention from those in an agitated flotation cell.

The spacing between openings is largely dictated by considerations ofadequacy of structural strength and the desired liquid and gas flowintroduction. Generally, each circular opening, is spaced from about0.25 to about 0.75 of the diameter of the opening from each other,typically about 0.5, although other arrangements are possible.Generally, the plurality of openings is arranged at a density of lessthan about 2 per square inch in a regular array.

The shroud 34 is illustrated a:s extending downwardly for the height ofthe impeller 28. It is possible for the shroud 34 to extend below theheight of the impeller 28 or for less than its full height, if desired.

In addition, in the illustrated embodiment, the impeller 28 is located adistance corresponding approximately half the diameter of the impeller28 from the bottom wall of the reactor 10. It is possible for thisdimension to vary from no less than about 0.25:1 to about 1:1 or greaterof the proportion of the diameter dimension of the impeller. Thisspacing of the impeller 28 from the lower wall allows liquid phase to bedrawn into the area between the impeller 28 and the shroud 34 from themass in the reactor.

By distributing the gases in the form of tiny bubbles and effectingshearing of the bubbles in contact with the iron chelate solution, rapidmass transfer occurs and the hydrogen sulfide is rapidly oxidized tosulfur. The reaction occurs largely in the region of the impeller 28 andshroud 34 and forms sulfur and hydrogen sulfide-depleted gas bubbles.

The sulfur particles initially remain suspended in the turbulentreaction medium but grow in the body of the reaction medium to a sizewhich enables them to be floated by the hydrogen sulfide-depleted gasbubbles. When the sulfur particles have reached a size in the range ofabout 10 to about 50 microns in diameter, they possess sufficientinertia to penetrate the boundary layer of the gas bubbles to therebyenable them to be floated by the upwardly flowing hydrogensulfide-depleted gas bubbles.

Other odiferous components of the hydrogen sulfide-containing gasstream, such as mercaptans, disulfides and odiferous nitrogenouscompounds, such as putrescenes and cadaversenes, also may be removed byadsorption on the sulfur particles.

At the surface of the aqueous reaction medium, the floated sulfuraccumulates as a froth 38 and the hydrogen sulfide-depleted gas bubblesenter an atmosphere 40 of such gas above the reaction medium 42. Thepresence of the froth 38 tends to inhibit entrainment of an aerosol ofreaction medium in the atmosphere 40.

A hydrogen sulfide-depleted gas flow outlet 44 is provided in the upperclosure 16 to permit the treated gas stream to pass out of the reactorvessel 12.

An adequate freeboard above the liquid level in the reaction vessel isprovided greater than the thickness of the sulfur-laden froth 38, tofurther inhibit aerosol entrainment.

Paddle wheels 46 are provided adjacent the edges of the vessel 12 inoperative relation with the sulfur-laden froth 38, so as to skim thesulfur-laden froth from the surface of the reaction medium 42 intocollecting launders 48 provided at each side of the vessel 12. Theskimmed sulfur is removed periodically or continuously from the launders48 for further processing.

The sulfur is obtained in the form of froth containing about 15 to about50 wt. % sulfur in reaction medium. Since the sulfur is in the form ofparticles of a relatively narrow particle size, the sulfur is readilyseparated from the entrained reaction medium, which is returned to thereactor 10.

The gas-liquid contact apparatus 10 provides a very compact unit whichrapidly and efficiently removes hydrogen sulfide from gas streamscontaining the same. Such gas streams may have a wide range ofconcentrations of hydrogen sulfide. The compact nature of the unit leadsto considerable economies, both in terms of capital cost and operatingcost, when compared to conventional hydrogen sulfide-removal systems.

There has previously been described in U.S. Pat. No. 3,993,563 a gasingestion and mixing device of the general type described herein. Inthat reference, it is indicated that, for the device described therein,if an increase in the rotor speed is made in an attempt to obtaingreater gas-liquid mixing action, then it is necessary to employ abaffle in the standpipe in order to obtain satisfactory gas ingestion.As is apparent from the description herein, such a baffle is notrequired in the present invention.

However, with larger size units designed to handle large volumes of gas,it may be desirable to provide a conical perforated hood structure abovethe impeller-shroud combination to quieten the surface of the liquidmedium in the vessel.

EXAMPLES.

Example 1

A pilot plant apparatus was constructed as schematically shown in FIG. 1and was tested for efficiency of removal of hydrogen sulfide from a gasstream containing the same.

The overall liquid capacity of the tank was 135 L. The standpipe had aninside diameter of 71/2 in., and the impeller consisted of six bladesand had a diameter of 51/2 in. and a height of 61/4 in. and waspositioned 21/4 in. from the base of the tank.

The pilot plant apparatus, fitted with a standard froth flotation shroudand impeller combination, was charged with 110 L of an aqueous solutionwhich contained 0.016 mol/L of ethylenediaminetetraacetic acid,iron-ammonium complex and 0.05 mol/L of sodium hydrogen carbonate. ThepH of the aqueous medium was 8.5. The shroud consisted of a stationarycylinder of outside diameter 12 in., height 5 3/4 in and thickness 3/4in. in which was formed 48 circular openings each 1.25 in. in diameter,for a total circumferential length of 188 inches.

Air containing 4000 ppm by volume of hydrogen sulfide was passed throughthe apparatus via the standpipe at a rate of 835 L/min. at roomtemperature while the impeller in the aqueous medium rotated at a rateof 733 rpm., corresponding to a blade tip velocity of about 211 in/sec.The gas velocity index through the shroud openings was 11.7 per secondper opening in the shroud. (The gas flow rate was 0.05 lb/min/opening.)Over the one and a half hour test period, 99.5% of the hydrogen sulfidewas removed from the gas stream, leaving a residual amount of H₂ S inthe gas stream of 20 ppm. Sulphur was formed and appeared as a froth onthe surface of the aqueous solution and was skimmed from the surfaceusing the paddle wheels. Simultaneous removal of hydrogen sulfide fromthe gas stream and recovery of the sulfur produced thereby, therefore,was effected.

During the test period, the pH of the aqueous solution dropped to 8.3but no additional alkali was added during this period. Further, noadditional catalyst was added during the period of the test.

Example 2

The procedure of Example 1 was repeated with an increased impellerrotation rate and higher gas flow rate.

Air containing 4000 ppm by volume of hydrogen sulfide was passed throughthe apparatus via the standpipe at a rate of 995 L/min. at roomtemperature while the impeller in the aqueous medium rotated at a rateof 1772 rpm corresponding to a blade tip velocity of about 510 in/sec.The gas velocity index through the shroud openings was 13.7 per secondper opening in the shroud. (The gas flow rate was 0.06 lb/min/opening.)Over the two hour test period 99.7% of the hydrogen sulfide was removedfrom the gas stream, leaving a residual amount of H₂ S of 11 ppm. Sulfurwas formed and appeared as a froth on the surface of the aqueoussolution and was skimmed from the surface. Simultaneous removal ofhydrogen sulfide from the gas stream and recovery of the sulfur producedthereby, therefore, was effected.

During the test period, the pH of the aqueous solution dropped to 8.3but no additional alkali was added during this period. Further, noadditional catalyst was added during this period of the test.

Example 3

The pilot plant apparatus was modified and fitted with a shroud andimpeller combination as illustrated in FIG. 2, was charged with 110 L ofan aqueous solution which contained 0.016 mol/L ofethylenediaminetetraacetic acid, iron-ammonium complex and 0.05 mol/L ofsodium hydrogen carbonate. The pH of the aqueous solution was 8.5. Theshroud consisted of a stationary cylinder of outside diameter 123/4 in.,height 81/2 in., and thickness 1/2 in. in which was formed 670 openingseach of 3/8 in. diameter for a total circumferential length of 789inches. Vertical baffles extending vertically from top to bottom of theshroud were provided on the internal wall equally arcuately spaced, tenin number with a 1/4-inch×1/4-inch space cross section. The impeller wasreplaced by one having a diameter of 61/2 in. The other dimensionsremained the same.

Air containing 4000 ppm by volume of hydrogen sulfide was passed throughthe apparatus via the standpipe at a rate of 995 L/min. at roomtemperature while the impeller in the aqueous medium rotated at a rateof 1754 rpm., corresponding to a blade tip velocity of about 597 in/sec.The gas velocity index through the shroud was 36.3 per second peropening. (The gas flow rate was 0.004 lb/min/opening.) Over the two hourtest period 99.998% of the hydrogen sulfide was removed from the gasstream, leaving a residual amount of H₂ S of less than 0.1 ppm. Sulphurwas formed and appeared as a froth on the surface of the aqueoussolution and was skimmed from the surface. Simultaneous removal ofhydrogen sulfide from the gas stream and recovery of the sulfur producedthereby, therefore, was effected.

During the test period, the pH of the aqueous solution remainedrelatively constant at 8.5. No additional alkali or catalyst was addedduring the period of this test.

As may be seen from a comparison of the results presented in Examples 1,2 and 3, it is possible to remove hydrogen sulfide with greater than 99%efficiency using an agitated flotation cell which is provided with aconventional shroud and impeller construction (Examples 1 and 2), asalready described in Canadian Patent No. 1,212,819. However, byemploying a higher blade tip velocity, as in Example 2, a modestincrease in efficiency can be achieved.

However, as seen in Example 3, with a shroud modified as describedtherein to provide the critical gas flow rate and using the criticalblade tip velocity, efficiency values over 99.99% can be achieved,leaving virtually no residual hydrogen sulfide in the gas stream.

Example 4

The pilot plant apparatus of FIG. 1 was tested for efficiency of removalof sulfur dioxide from a gas stream containing the same. The elements ofthe pilot plant apparatus were dimensioned as described in Example 3.

The pilot plant apparatus was charged with 110 L of an aqueous slurrycontaining 13.2 kg of CaO and 3450 g of MgSO₄.7H₂ O. Air, containingvarying amounts of sulfur dioxide was passed through the apparatus viathe standpipe at varying flow rates at room temperature, while theimpeller in the aqueous slurry rotated at a rate varying from 1760 to1770 rpm, corresponding to a blade tip velocity of 599 to 602 in/sec.The corresponding gas velocity indices through the shroud were from 31.1to 124.5 per second per opening. (The gas flow rates were 0.003 to 0.01lb/min/opening.)

A series of one hour runs was performed and the residual SO₂concentration was measured after 45 minutes. The results obtained areset forth in the following Table I:

                  TABLE I                                                         ______________________________________                                                     SO.sub.2 Concentration                                           Gas Flow Rate                                                                              (ppmv)                                                           (cfm)        In *.sup.1  Out *.sup.2                                                                           RPM                                          ______________________________________                                        30           1000        <0.4    1760                                         30           5000        <0.4    1760                                         30           7000        <0.4    1760                                         30           10000        0.6    1760                                         60            900        <0.4    1770                                         75           1000        <0.4    1760                                         100          1000         0.8    1763                                         120          1000         5.6    1770                                         ______________________________________                                         Notes:                                                                        .sup.1 Concentration values vary approximately ± 10%.                      .sup.2 Concentration values vary approximately ± 0.2 ppm by volume.   

As may be seen from this data, highly efficient (>99.99%) removal ofsulfur dioxide from the gas stream was obtained using a lime slurry,even at high sulfur dioxide concentrations and less efficient removalwere observed only at high gas flow rate.

Example 5

The procedure of Example 4 was repeated using 110 L of an aqueous slurryof 13.2 kg of calcium carbonate and 3450 g of MgSO₄.7H₂ O. In theseexperiments, the impeller was rotated at a speed of 1770 to 1775 rpm,corresponding to a blade tip velocity of 602 to 604 in/sec. Thecorresponding gas velocity index through the shroud were 31.1 to 103.8per second per opening. (The gas flow rates were 0.003 to 0.01lb/min/opening)

The results obtained are set forth in the following Table II:

                  TABLE II                                                        ______________________________________                                                     SO.sub.2 Concentration                                           Gas Flow Rate                                                                              (ppmv)                                                           (cfm)        In *.sup.1  Out *.sup.2                                                                           RPM                                          ______________________________________                                        30            900        <0.4    1770                                         30           2000        <0.4    1770                                         30           3000        <0.4    1770                                         30           5000        <0.4    1770                                         30           9000        <0.4    1770                                         30           10000       <0.4    1770                                         45           1000        <0.4    1773                                         60           1000        <0.4    1775                                         75           1050        <0.4    1775                                         100          1000         5.25   1775                                         ______________________________________                                         Notes:                                                                        .sup.1 Concentration values vary approximately ± 10%.                      .sup.2 Concentration values vary approximately ± 0.2 ppm by volume         except for last run, approximately ± 1 ppm by volume.                 

As may be seen from this data, highly efficient (>99.99%) removal wasobtained using a limestone slurry, even at high sulfur dioxideconcentrations and less efficient removal were observed only at high gasflow rate.

SUMMARY OF DISCLOSURE

In summary of this disclosure, the present invention provides novelmethod and apparatus for effecting gas-liquid contact for removal ofcomponents from gas streams, such as by chemical reactions or physicalseparation and, if desired, for separating flotable by-products of suchreactions using an agitated flotation cell, modified in certain criticalrespects to function as an efficient gas-liquid contactor. Modificationsare possible within the scope of this invention.

What we claim is:
 1. A gas-livid contact apparatus, comprising:enclosedtank means, inlet gas manifold means for feeding at least one gas streamthrough an inlet in an upper closure to said tank means, standpipe meanscommunicating with said inlet and extending downwardly within said tankfrom said upper closure, impeller means comprising a plurality of bladeslocated towards the lower end of said standpipe means and mounted to ashaft for rotation about a generally vertical axis, drive means forrotating said shaft, shroud means surrounding said impeller means andhaving a plurality of openings extending through the wall of said shroudmeans, each of said openings through said shroud means having anequivalent diameter of less than about 1 inch and such that the ratio ofthe equivalent diameter to the diameter of the impeller is less thanabout 0.15, and vent means from said tank means.
 2. The apparatus ofclaim 1 wherein said shroud has a diameter corresponding to about 2:1 toabout 1.2:1 times the diameter of the impeller.
 3. The apparatus ofclaim 2 wherein said impeller has a height corresponding to about 0.3:1to about 3:1 times the diameter of the impeller.
 4. The apparatus ofclaim 3 wherein said impeller has at least 4 equally-angularly spacedblades and said shroud has a diameter which is approximately 1.5 timesthat of the impeller.
 5. The apparatus of claim 2 wherein said standpipehas a diameter corresponding to about 1:1 to about 2:1 times thediameter of the impeller and said shroud has a diameter not less thanthat of said standpipe.
 6. The apparatus of claim 5 wherein saidimpeller is spaced from a bottom wall of the vessel at least 0.25 timesthe diameter of the impeller.
 7. The apparatus of claim 1 wherein saidplurality of openings is arranged to provide a gas flow rate of about0.005 to about 0.007 lb/min/opening in the shroud.
 8. The apparatus ofclaim 7 wherein each of said openings is of circular shape.
 9. Theapparatus of claim 8 wherein the openings are spaced about 0.25 to about0.75 times the diameter of the openings from each other.
 10. Theapparatus of claim 9 wherein the openings are spaced about 0.5 times thediameter of the opening from each other.
 11. The apparatus of claim 8wherein the plurality of openings is arranged at a density of less thanabout 2 per square inch in a uniform array.
 12. The apparatus of claim 1wherein each of said openings is dimensioned with an equivalent diameterof from about 3/8 to about 5/8 in.
 13. The apparatus of claim 12 whereinsaid openings are all circular of the same diameter.
 14. The apparatusof claim 13 wherein each said opening in said shroud has sharp edges atits upstream and downstream ends.
 15. The apparatus of claim 1 whereinsaid shroud is of right cylindrical shape.
 16. The apparatus of claim 1wherein said shroud is stationary.
 17. The apparatus of claim 1 whereinsaid plurality of openings is arranged to provide a gas velocity indexof at least about 18 per second per opening in said shroud.
 18. Theapparatus of claim 17 wherein said gas velocity index is at least about24 per second per opening in said shroud.
 19. The apparatus of claim 18wherein said gas velocity index is from about 40 to about 400 per secondper opening in said shroud.
 20. The apparatus of claim 1 wherein saidshroud has a plurality of baffles on the internal wall thereof extendinggenerally axially of the shroud.